The IPTV service is a general term of services which are provided to users by an IP bearing network and can support an interactive live TV program, Video on Demand (VOD), time-shifting broadcasting and other various application-based services. Due to the appearance of the IPTV, the traditional telecom operators see a new service growth point, and can integrate their advantageous network resources to explore an innovative service and an innovative commercial profit model without being limited by the provision of voice services and network bandwidths any more. In addition, the traditional Radio and Television operators can also take the IPTV as a new TV program marketing mode to sell a lot of idle program resources, so that the IPTV will provide a new opportunity for the development of the Radio and Television.
The IPTV program is generally transmitted in a multicast mode. The multicast is a network technology which allows one or more senders (multicast sources) to once send a single data packet to multiple receivers synchronously. The multicast source sends a data packet to a specific multicast group, and only an address belonging to the multicast group can receive the data packet. In the IPTV, generally, there is only one multicast source; even with a doubled number of users, the backbone bandwidth does not need to increase as any one of backbone links in the whole network only transmits a single video stream no matter how many target addresses there are, i.e., the so-called “once transmission for group broadcasting”. The multicast improves the data transmission efficiency and reduces the congestion possibility of the backbone network. For an IPTV system, the number of users supported by it is only limited by the capability of an access network and has nothing to do with the pushing capability of a front-end push system.
The two key points for the multicast are selections of a multicast control point and a multicast replication point. The multicast control point completes determining on authority of a user to accept multicast data. When a user opens an IPTV service, a user management module in the IPTV system stores order information of the user; after the user selects a channel, a Set Top Box (STB) device requests to join in a multicast group corresponding to the channel by transmitting an Internet Group Message Protocol (IGMP) join request and other messages; and the multicast control point, after accepting the request, determines whether the user has the authority to watch the channel according to the order information multicast authority table of the user delivered by the user management module and can also authenticate the user by the Radius and other protocols. The multicast replication point replicates multicast streams to a port by determining whether the port has a multicast receiving request and corresponding user authority, and under the multicast replication point a live program is transmitted to the user in a unicast mode (such as Point-to-Point Protocol over Ethernet (PPPoE)), therefore the multicast replication point is more adjacent to User Equipment (UE), more network bandwidths are saved. In order to reduce the interaction complexity between the multicast replication point and the multicast control point and ensure intercommunication between the multicast replication point and the multicast control point, the multicast replication point and the multicast control point can be arranged on the same equipment. In combination with the existing condition of the Metropolitan Area Network (MAN) of the operators, the multicast replication point is generally arranged on a Digital Subscriber Line Access Multiplexer (DSLAM)/Broadband Remote Access Server (BRAS)/switch; and it is required that these devices at the network side should support the multicast, and the access network switch and other devices should support the IGMP snooping protocol, the IGMP proxy protocol and other protocols so as to implement controllable forwarding of the multicast data. A device with the multicast replication point should support a flexible Virtual Local Area Network (VLAN) function, bears different services such as the multicast, internet surfing and voice by different VLANs, and ensure a flexible Quality of Service (QoS) management capability and higher security for the system.
The main testing indexes for the IPTV service performance include a maximal pushed channel number, a average channel bandwidth, average channel switching time, I frame joining delay, average channel leaving delay, average channel joining delay, channel overlapping time and channel interval time, wherein
(1) the maximal pushed channel number is the maximal number of channels which can be provided by a streaming media system, a user can traverse all the channels within a short time;
(2) the average channel bandwidth refers to the bandwidth occupied by each channel program. The bandwidth occupied by a Standard Definition Television (SDTV) program and a High Definition Television (HDTV) program are different; and the bandwidth occupied by the HDTV program is much higher than that occupied by the SDTV program;
(3) the average channel switching time refers to the time from the moment when an IGMP leaving message is transmitted to the moment when the first multicast packet of the requested channel is received;
compared with the channel switching of the IPTV which can only be implemented by the IGMP, the channel switching of traditional TV is much faster; the channel switching time of the IPTV should be generally within 2s, otherwise, the time of waiting for the channel switching is unbearable to the user; at present, how to solve the too slow channel switching of the IPTV is an urgent and main task. The IGMP is mainly processed by an access device, therefore the multicast replication and processing capability of the access terminal including the multicast switch, the multicast router, the DSLAM and other devices are important factors of affecting the channel switching time;
(4) the I frame joining delay: the successful transmission of a request for watching programs from a client does not mean that video streams have been transmitted to the receiving terminal of the user; and there are three kinds of video frames: I frames bearing information of the whole image, P frames calculated by taking the I frames or P frames as an reference, and B frames calculated by taking the I frames or P frames as an reference; only when the receiving terminal receives the I frames can indicate that the user sees the video program and the channel is really switched successfully; therefore, obtaining the I frame joining delay is more significant to the test of the channel switching time;
(5) the average channel leaving delay refers to the time from the moment when the IGMP leaving message is transmitted to the moment when the last multicast packet of the channel is received, generally, the shorter channel leaving delay is desired;
(6) the channel overlapping time refers to the time interval from the moment when the first multicast packet of the requested channel is received to the moment when the last multicast packet of the original channel is received if the method that the multicast packet of the original channel is not stopped until the multicast packet of the requested channel is received is adopted during switching of system channel;
(7) the channel interval time refers to the time interval from the moment when the last multicast packet of the original channel is received to the moment when the first multicast packet of the requested channel is received if the method that the multicast packet of the original channel is stopped prior to the receiving of the multicast packet of the requested channel during switching of system channel.
Whether the IPTV service is developed smoothly is determined by final service experience of users, which is affected by the delay in the IPTV service. Therefore, to obtain the QoS acceptable to users, the delay in the IPTV service must be as less as possible.